I'm not a plumbing expert, but if you are getting bursts of high pressure when you move the selector valve. I would suspect that is where the problem is. You may have somethng either stuck in there or the valve has a problem. I don't how hard it is to take the valve out of an Essex but thats' where I would start. Good luck!
Tony & Ruth........... FMCA#F416727
2016 London Aire 4519, Freightliner chassis, Cummins ISX, 2018 Jeep Wrangler Unlimited, Blue Ox Avail with AF1. TST 507 TPMS